Biomedical Engineering Reference
In-Depth Information
Table 4 Reference
techniques used for
correlating dielectric
spectroscopy signals
Technique
References
Cell counts and staining methods
[
61
,
70
,
71
]
Cell diameter, size
[
56
,
58
,
59
,
73
]
Colony-forming units (CFU)
[
61
]
Cross-sectional area
[
58
,
59
]
Dry cell weight (DCW)
[
43
,
54
,
57
,
61
,
66
,
90
]
DNA
[
54
]
NTP
[
68
]
Packed (mycelium) volume
[
57
,
61
,
66
]
OD/turbidity
[
43
,
57
]
Viscosity
[
54
,
55
]
2.2.3 Reference Methods for Use in Dielectric Spectroscopic Correlations
The quest for a reference standard for the correlation of the capacitance reading to
relevant biological information is still ongoing, as highlighted in Chap. 1.
Regardless of the nature of the cells considered, the capacitance of a lipid mem-
brane is on average 0.5-1 lF per cm
2
of membrane area [
45
]. The capacitance
measurement still needs to be transformed into relevant information depending on
the aim of the study. Research groups have described several correlation methods
over the past two decades (Table
4
). Xiong et al. [
61
] compared the most common
techniques, namely optical density at 600 nm, dry cell weight (DCW), packed
mycelial volume and colony-forming units (CFU), and obtained linear correlations
in the range 3.1 9 10
6
cell/mL to 9.2 9 10
9
cells/mL for Saccharomyces cere-
visiae with all techniques. Cell counting methods, especially when associated with
Trypan blue staining to differentiate between viable and non-viable cells, are
commonly used to correlate capacitance measurements to cell number or cell
viability for animal cells. However, if crystal violet is used for nuclei counting,
deviations from linearity can be observed if cells tend to be multi-nucleated under
specific culture conditions [
70
]. Neves et al. [
54
] observed linear correlations of
capacitance with DNA, packed mycelial volume and CO
2
production during the
exponential growth phase, while correlations between DCW and capacitance
measurements tended to be linear during exponential, transition and stationary
phases. Correlations tend to be linear as long as no metabolic, physiological or
morphological changes occur within the culture [
61
]. Indeed, as mentioned pre-
viously, dielectric spectroscopy is not only a tool to measure viable cell volume,
but also to detect changes during biotechnological processes [
73
,
84
]. Maskow
et al. discussed the reasons and implication of non-linear correlations between
biomass and capacitance correlations in the scope of bioprocess monitoring and
control [
59
].
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